Ground speed indicator utilizing doppler effect



E. 1. ANDERSON 2,455,639

GROUND SPEED INDICATOR UTILIZING DOPPLER EFFECT Filed 001:. 26, 1943 Dec. 7, 1948.

tau/YE? Y Zhmeutor E ARL L'ANDERSUN Gttomeg UNITED/ STATES PATENT I oF lcE GROUND SPEED INDICATOR UTILIZING DOPPLER EFFECT Earl I. Anderson, Manhasset, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application October 28, 1943,Serial No. 507,741

, l My invention relates to systems for determining the relative speeds of objects and particularly to a system in which the Doppler effect is utilized for obtaining the speed or drift ofan aircraft with respect to the earth. A system of the above-mentioned type is described and claimed in a copending application r Claim. (01. 34H) Serial No. 352,845, filed August 16, 1940, in the I name oi Irving Wolff, and entitled Drift indicators, now Patent 2,403,625 issued July 9, 1946. The present invention is an improvement on the system described in the Wolf! patent.

One object of the invention is to provide an improved receiver for a speed indicator system wherein radio waves are transmitted in different.

directions, received from these dlrectionsafter reflection, and then reduced in frequency and beat with each other to determine 'the Doppler frequency.

Another object of the invention is to provide in a speed or drift indicator an improved method of and means for obtaining, an indication of the Doppler frequency.

In one preferred embodiment of the invention pulses of radio energyare radiated toward the earth forwardly and rearwardly from an alrcraft, and the pulses reflected from the earth at the front and at the rear of "the aircraft are supplied to separate detectors and heterodyned to a lower frequency by means of a single oscilpulses of suflicient amplitude to operate the counter reliably" The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing in which the single figure is a circuit and block diagram of a system embodying the invention.

Referring to the drawing, radio pulses are radiated forwardly and downwardly from a directive antenna in andtsimilar pulses are radiated rearwardly and downwardly from ,a di-,

. rective antenna Ii. The reflected pulses may be received by the same antennas l0 and II by utilizing suitable means for short-circuiting the lines to the receivers during the instant of pulse however, the use of separate receiving antennas will be-assumed. a

' As will be explained hereinafter, only the pulses reflected from preselected points such as those later which is common to the two detectors. The

oscillator may be oi a type having tuned plate and grid circuits, the plate circuit being coupled to one detector and the grid circuit being coupled to the other detector. Since the oscillator plate and grid circuits are substantially isolated from each other, the signal from one detector will not be fed through the common oscillator to the other detector to produce undesired beats. After being reduced in frequency the fore and aft reflected signals are supplied to a detector or mixer tube to obtain pulses that vary in amplitude at the Doppler or beat frequency rate.

In accordance with the present invention, the

' output of the above-mentioned mixer tube is applied to a tuned circuit that is. resonant at the pulse repetition rate whereby there is ob-' 45 degrees ahead and behind the airplane ,are passed to the receivers and utilized in measuring the Doppler effect, this directional selection-being accomplished bythe use of gating, .pulses as described in the above-identifled Wolff application. I It may be noted that such gating is de-, sirable because it isdiflicult to provide antennas that are suitable for installation on air-. craft and which have a sufliciently narrow radi; ation pattern.

The antennas Ill and II. (which are shown as dipoles in parabolic reflectors merely by way of example), are supplied with short pulses of'high frequency radio energy from a transmitter l2. Such pulses-may have a carrier'frequency of 400 megacycles, a duration of about 0.3 microsecond, and a repetition rate of 15,000 per second.

These pulses of high frequency energy are obtained by means of a 15,000 cycle sine wave oscillator I: which is coupled/to a pulse generator I4. The generator l4 produces short 15,000 cycle modulating pulses I! for pulse modulating the transmitter l2.

Output signal from the oscillator I3 is also passed-through a phase shifter l6 and supplied to-a pulse generator i'l for producing time gate pulses ,l I. Thepulses it have the same repetition rate as the modulating pulses II and may be of substantially the same duration. In the ex-- ample being described they have a duration of 0.25 microsecond. Their use for obtaining directional selection will be described hereinafter.

The receiving system comprises directive rea are supplied through an 8 ceiving antennas 2i and if for receivlngreflected pulses that have been radiated from the antennas II and II, respectively. The signals from antennas II and I! are supplied to the flrst detectors it and 24, respectively, where they are mixed with signal from a common local oscillator It to produce intermediate-frequency signals.

names of Earl I. Anderson and Allen Barco, and

entitled Ground speed indicator. ,now Patent 2,422,064 issued June 10, 1947. In this way the detectors it and 24 are isolated with respect to each other even though they are supplied with mixing signal from an oscillator common to the two detectors. By employing a single oscillator in this way any difilculty resulting from changes in oscillator frequency is avoided. While it is feasible to lock together two comparatively low frequency oscillators as described below so that anyfrequency change in one will cause a corresponding frequency change in the other, this cannot be done satisfactorily with present oscillators operating at very high frequencies, such as frequencies in the region of 400 megacycles per second. I

The I.-I".'outputs from the first detectors II and 24 (which outputs will differ idirequency only by the amount of the Doppler are passed through L-F. amplifiers 4i and 42 to the second detectors 43 and 44, respectively. Here the 1-1 signals are again heterodyned 'tojaflower' frequency by means of the oscillators 4i and 41 which are coupled to the detectors 4! and u respectively, In order to hold the oscillators, 44

and 41 exactly to the same frequency, each locked in with the other gether through a common v I this point in the circuit this may be 'done satis factoi-iiy because of the lower frequency involved.

The I.-F. outputs of the second detectors 43 by m itm nter; inductance colljlt nt is positively biased beyond cut-of! by means of a connection" to a voltage divider 51, there being a bypass capacitor Ill connected between cathode and ground. A time gate pulse ll of negative polarity is applied through a coupling capacitor II to the cathode of a pentode 89 to reduce its bias to the cut-off point during the pulse interval. It

therefore operates as a time gate detector because it is in operative condition (at cut-ofl D.-C. bias) only when the pulse is applied and is substantially beyond cut-oil at all other times.

The I.-F. pulses l5 corresponding to the fore and aft reflected pulses are applied from the amplifier 51 to the control grid of the pentode ll through a coupling capacitor 1 i These pulses are reflected from the earths surface within a wide' angle rather than from only a small surface area and along the two'45 degree lines only. Thus a reflected pulse appears at the receiver input as a pulse of comparatively'long duration. The grid circuit of pentode 59 includes an inductance coil 12 for-providing coupling impedance, the coil I! being shunted by a resistor 13. An oscillation suppressor resistor 14 may be provided.

In the absence of a gate pulse II, the tube 44 is biased beyond cut-off by a positive cathode bias that is applied from the voltage divider 51 through a cathode resistor I6. flected pul'ge signal that passes through the tube 58 is that which appears on itscontrol grid simultaneously with the appearance of a gating pulsell on its cathode. By adjustment of the phase shifter It the pulse I! can be made to occur at the proper time to pass only those reflected pulses that have traveled a certain specific distance, this distance being that along the 45 degree line in the particular example assumed.

Since the pentode i9 is to function as a detector for obtaining the Doppler or beat frequency of the, fore and ,aft reflections, its cathode bias and the amplitude of the gate pulses it are properly adiusted to make the tube 59 act as'a detector-during the occurrence of each pulse l8.

and 44 are passed through L-I". amplifiers 4| and 5i and through amplitude limiters 52 and I}, respectively. The outputs of the limitea 5:oandail as.

tector II which also has the time ga applied thereto.

The two 1.4. siBflals beat with each other in the detector 58 whereby .the detector output has a beat frequenc'y that is the frequency difference resultin'gxfrom the Doppler effect. The gating pulses I! allow the detector "to pass only the signals reflected along the desired fore and aft'45 degree-angle lines.

The output of detector" is applied to a circuit ll tuned to the 15,000 cycle pulseirepetition rate and then passed through rectifier" to obtain pulses of current occurring at the beat or Doppler frequency. The Doppler frequency pulses are then supplied to a suitable frequency indicating or counter circuit I. Thecounter circuit 52 may be'of'the type commonly employed in radio altimeteis of the frequency modulated type. The function and advantages of the tuned circuit" will be described hereinafter.

The keyer tube 2| comprises a pentode having in its grid circuit an" adjustable inductance coil .ll'to varythe pulse width and a resistor 44 for suppressing parasitic oscillations. The cathode from the ampi ia rflii effect frequency; It should be understood that;

The beat frequency output of the tube 44 is shown by the graph 15 and consists of the 15,000 cycle video pulses which vary in amplitude at the Doppler or beat frequency. These pulses are ill)- plied through a coupling capacitor II to the circuit 59 which istuned to the pulse repetition rate of 15,000 cycles per second. Thus, as shown by the graph 18, there appears across the tuned circuit 59 a sine wave voltage of 15,000 cycles which varies in amplitude at the rate of the Doppler the Qof the tuned circuit 58 should be low enough to permit the amplitude of the sine wave voltage to follow the amplitude variations of the modulated pulses that are applied thereto.

' The beat frequency modulated sine wave II is rectified by a diode or other suitable rectifier ii whereby there appears across the output resistor I9 the rectified signal 5 i The signal II is amplifled by a triode 82 and integrated by a resistor- I capacitor filter 83 to produce pulses 84 that recur at the Doppler frequency. The pulses 84 are then shaped (clipped) by a vacuum tube 45 and sup plied to a suitable indicator or pulse counter 42.

By utilizing the tuned circuit II, a large output is obtained since the narrow pulses of small energy content shown in graph 15 are converted by the tuned circuit 59 into voltage sine waves whose peak amplitude is still about as great as though a resistance load were used'but whose integrated area is greater. Thus the usable energy output from the very narrow modulated Therefore, the only revfor supplying heterodyning signals of pulses is increased by the tuned circuit I9 al-- though the tuned circuit actually introduces. the

- usual circuit loss. The invention may also be employed to advantage in other systems, such as a communication system, where it is desired to demodulate very narrow recurring pulses of small energy content that have been modulated either as to amplitude or as to width.

The use of the tuned circuit 59 alsoimproves the signal-to-noise ratio as compared with that obtained when the tuned circuit -59 is replaced by a high impedance resistor. This is because the circuit 59 has low impedance to noise or like !re- 25 quency to said detectors in the two channels for producing intermediate-frequency signals in said two channels, means for mixing said two intermediate-frequency signals to obtain pulses modulated by their beat frequency, a circuit which is resonant at the pulse repetition rate, means for applying said modulated pulses to said resonant circuit wherebyla beat-frequency modulated sine wave signal is obtained, and means for rectifying said sine wave signal to obtain pulses recurring at the beat frequency.

EARL I. ANDERSON.

REFERENCES- CITED The following references are of record in the tile of this patent:

UNITED STATES PATENTS Number Name Date 1,545,940 Cabot July 14, 1925 1,564,627 Round Dec. 8, 1925 2,381,928 Roberts Aug. 14, 1945 2,403,625 Wolf! July 9, 19.46 2,418,286

Busignies Feb. 25, 1947 

